1. Field of the Invention
This invention concerns a method and apparatus for forming color images by means of electrophotography.
2. Discussion of the Background
Conventionally, when forming color images by means of electrophotography, a photosensitive drum and a transfer drum carrying recording paper are made to rotate together a plurality of times, and images on the photosensitive drum are transferred the same number of times onto the recording paper on the outer surface of the transfer drum so as to overlay each other.
FIG. 1 is a simplified drawing of a color electrophotographic copier employing the conventional color image-forming method (as shown for example in Japanese Patent Application Publication No. 34467/1988; and Proceedings of the Inst. of Electrostatics Japan, Vol. 9, No. 4. pp. 253-261, 1985).
In the figure, the surface of a photosensitive drum 1 which rotates at a fixed speed in the counterclockwise direction, is electrostatically charged with a specified polarity by a corona discharge device 2.
When an image of the original is formed by the exposure system 3 on the electrified surface of the photosensitive body, the charge on the photosensitive body is lost according to the density of the respective parts of the image of the original, and electrostatic latent images are thereby formed. These latent images are then rendered visible by a developing device 4. In the case of a color copier, this developing device 4 comprises at least three devices, namely a device for developing yellow images 5, a device for developing magenta images 6, and a device for developing cyan images 7.
The exposure device 3 is therefore provided with blue, green and red color separation filters. The developing device 4 may also be provided with a device for black development in order to adjust the color tone if necessary.
Developing is performed for each color separately. First, when a color image of the original is projected on the photosensitive body using the blue filter, an electrostatic latent image consisting of a yellow element which is the complementary color to blue is formed thereon. This is then rendered visible by the yellow developing device 5. The yellow developing device 5 contains a developing agent of a yellow toner and an iron powder carrier. These two components are mixed together with stirring, and the toner is triboelectrified with a polarity opposite to that of the charge on the electrostatic latent image. Both the toner and carrier are attached onto the developing roller 5a, and are transported to the developing area which is near the photosensitive drum 1.
Next, the green and red color separation filters are selected, the magenta and cyan toners are developed, and the toner images are copied onto a recording paper 9 via a transfer drum 8 to form color images on the paper.
However, in the conventional color image-forming method, the outer circumference of the transfer drum 8 must be arranged to be longer than the length of the recording paper 9.
If for example the largest size of the recording paper 9 which can be recorded is B4 (JIS Standard), its dimension in the longer direction is 364 mm. In order that the recording paper of this B4 size can be wound onto the transfer drum 8 without superposing, the outer diameter of the transfer drum 8 must be not less than 116 mm.
Because a mechanism required to hold the recording paper 9 at a fixed point must also be provided on the circumference thereof, the transfer drum 8 must have larger dimensions than the above, and the color copier becomes bulky.
Further, the mechanism required to load the recording paper 9 on the transfer drum 8, form a recorded image on the paper 9, and release the paper 9, is complex. The cost of the copier therefore increases, and the paper frequently jams.
Moreover, in the above conventional method of forming color images, costly and bulky color separation filters are required, and a means of selecting them must also be provided.